Thorium-Argon hollow cathode lamps are commonly used as wavelength calibration lamps for high-resolution astronomical spectrographs (e.g. the European Southern Observatory’s (ESO) High Accuracy Radial velocity Planet Searcher (HARPS) and Ultraviolet and Visual Echelle Spectrograph (UVES)). They have been instrumental in supporting high precision work such as the search for extra-solar planets using the radial-velocity method. However, several years ago astronomers found that the quality of commercial Th-Ar lamps had deteriorated, when new lamps showed a “forest” of lines at low intensity levels obscuring faint atomic thorium lines rendering them useless for wavelength calibration in some regions. Based on information provided by the manufacturers the presence of molecular emission from thorium oxides has been suspected as the likely cause of this problem. We have now conclusively identified the observed emission bands as being due to strong molecular bands of ThO, confirming the source of the contamination of the hollow cathode lamps.

We have recorded spectra of new Th-Ar lamps showing contamination using the high-resolution echelle spectrograph at the University of Wisconsin and ESO’s new Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) spectrograph in order to determine the positions of the ThO lines. We shall present our initial analyses of these spectra, describing the wavelength regions most affected, their dependence on the lamp current, and measurements of the ThO line positions.